The invention replaces the customary belt-tension control of continuously variable belt transmissions (CVT) with electro-hydraulic actuation. A transmission construction of this kind is shown, in section, in FIG. 1. The engine torque M.sub.m output by the internal combustion engine 1 can be influenced by means of the throttle flap 2. The throttle flap 2 is coupled, for example, mechanically or electrically to the accelerator pedal (not shown). The internal combustion engine 1 is generally coupled to the drive (primary) end of the continuously variable transmission 4 by means of a clutch and/or a converter 3. The output (secondary) end of the continuously variable transmission 4 is connected to the wheels of the vehicle via a transmission (not shown) connected downstream of the transmission 4. On the primary and secondary ends, the continuously variable transmission has respective axially displaceable conical pulley discs 5 and 6. To adjust the transmission ratio, a corresponding primary pressure P.sub.p and a secondary pressure P.sub.s are built up in the respective oil chambers 7 and 8. It must be ensured by a suitable choice of the manipulated variables constituted by the primary pressure P.sub.p and the secondary pressure P.sub.s that:
1. the transmission ratio i corresponds to the desired ratio between primary rotational speed N.sub.p and secondary rotational speed N.sub.s ; and, PA1 2. the force-transmitting belt 9 (or chain, band) is pressed sufficiently hard against the pulleys to prevent the belt 9 from slipping. PA1 1. The slipping of the operative means, generally a force transmitting belt or a band or chain, is reliably prevented. In the known solutions, the pressure must be fixed at a level high enough to ensure that slipping is reliably prevented. Unevennesses in the road, braking operations, downshifts and brief increases in the torque to be transmitted due to dynamic load fluctuations can cause problems, however, making additional measures to avoid slipping necessary. PA1 2. The transmission operates with a higher efficiency. PA1 3. The mean pressure level and thus the hydraulic losses are reduced. PA1 4. The requirements on the accuracy of the subordinate pressure control system are lower.
The above-mentioned point 1 is achieved by means of an electrohydraulic transmission ratio control or primary rpm control 10. For point 2, a belt-tension control 11 is provided.
Rotational-speed sensors 12, 13 and 14 are provided on the engine 1 and on the continuously variable transmission 4 for transmission-ratio and belt-tension control and these sensors detect the engine rpm N.sub.m, the primary rotational speed N.sub.p and the secondary rotational speed N.sub.s.
In the frequently used master-slave system shown in FIG. 1, the secondary pressure P.sub.s is used to set the belt tension and the primary pressure P.sub.p to set the transmission ratio or primary rotational speed. In the alternative partner system, the belt-tension control system influences both the primary and the secondary pressure.
In general, it can be stated that an actuating signal for the belt-tension control is available in the form of a pressure variable P.sub.B. A number of methods for controlling the belt tension are known from the literature but these all operate in a similar manner. In the following, EP-A1-0,451,887 is used for the purpose of explanation: for this purpose, FIG. 2 shows the belt-tension control system described in this patent publication. The characteristic field 34 is used to determine the outputted engine torque T.sub.m from the throttle-flap angle .beta. and the engine rpm N.sub.m. Blocks 38 and 35 can be used to calculate the primary torque T.sub.pr output by the converter. Block 45 calculates the transmission ratio of the continuously variable transmission. By means of block 46, the secondary torque T.sub.sec can be estimated from the transmission ratio and the primary torque T.sub.pr. By means of block 49, the secondary radius R.sub.sec is calculated from the transmission ratio. The minimum secondary pressure P.sub.sec required is then derived from the secondary radius and the secondary torque T.sub.sec by means of block 52. After the addition of a reserve pressure in block 64, the signal is smoothed by means of a low-pass filter 68. The mean pressure P.sub.sec (CENTR) generated by centrifugal forces at the secondary pulley is then subtracted from the above pressure by means of block 60. In addition, the result for the pressure 63 is influenced by a sensor signal for the measurement of ground unevenness 94 and is then limited to a selectable range by a limiter 168. The result of the calculations P.sub.sec (TV) thus far is then used further by a subordinate pressure control circuit 70, 72, etc. The pressure control circuit ensures that the pressure P.sub.sec (MV) at the secondary pulley is set to the value P.sub.sec (DV).